Assisted Migration

“Assisted migration” is the human-assisted movement of organisms to new environments. It has been used in forestry and conservation biology to hedge against the effects of climate change. In simplest terms, plants from warmer climates are moved to cooler climates in anticipation of future warming.

The application of assisted migration has stimulated much debate in the scientific community. Detractors adhere to traditional notions that “local is best” and point out the possibilities of failure — and even harm. Proponents point out the risks of extinction and loss of economic value for those species that are unable to adapt to a changing climate without human assistance.

Little of the scientific literature on assisted migration focuses on home landscaping (the focus of the plant sync) — but the attendant controversy may be moot in this sector. Urban and residential landscapes have used plants from outside of their historic range for centuries. This trend is amplified in modern times by a widespread commercial nursery industry and global commerce (See Van der Veken et al., 2008 below). Selection and breeding programs continuously create plant varieties for ornament that are quite different from their native progenitors. Neither of these facts are likely to change within a time window in which climate change action would be effectual, and, broadly speaking, there is no functional reason they should. The question instead is how to manage the diversity of plants in home landscapes to optimize ecosystem services and avoid environmental harm — regardless of genetics or provenance. The provision of ecosystem services by plants in urban and residential environments is not a new idea, but deploying plants in consideration of climate change (e.g. assisted migration) is not a major topic in the ornamental horticulture scientific community, the industry, and the gardening public.

Ornamental plants are chosen for urban and residential landscapes for beauty, shade, wildlife, pollinators, noise reduction, pollution remediation, etc. etc. Shall we make climate change resilience, mitigation, and adaptation a prominent member of this list? And can assisted migration — mindful of risks — play a role?

An Ode to Provenance – On the occasion of Mom’s 81st birthday

Happy birthday Mom and Dad.

My Mom (1935) and Dad (1929) were born on the same date, six years apart. Shortly after moving into their Tennessee home in 1964, Dad dug a small oak seedling up from somewhere on the property and planted it in the new construction debris that made up our front yard (Despite the impoverished site, it became a majestic tree; until a truck rumbled through a few years ago and cut it in half lengthwise to keep it out of the power lines. But hope springs eternal — as we shall see). Upon Dad’s passing in 2000, Mom put a small plaque on the tree and sprinkled his ashes around. Apart from beauty and shade, the tree was now . . . important. With this newly christened treasure, I set about to preserve it for all time.

So how does one immortalize an oak tree? Why, collect the acorns, of course. Plant ’em. Help ’em grow.

So I did. The result is three young trees in Ankeny, IA , the largest of which is a strapping thirty-foot teenager. Despite its Tennessee provenance it has thrived through Iowa’s extreme winters with no trouble at all. Dad would be proud.

As we proceed further into a changing climate, these offspring oaks, in Iowa ground and of Tennessee provenance, may have more to give. Iowa may become progressively more like Tennessee (map). Some plant species adapted to Iowa’s past and current climate will struggle or fade away. Indeed, the very notion of “native” species will be turned on its head: what does “native” mean when the “native” environment is repeatedly swept away on timescales of a single human lifespan? The assumption (by some) that “native” plants and plant communities are always the most suitable for urban and home landscapes becomes more uncertain.

“Resilient” and “functional” will come to more accurately describe plant specimens, species, and communities that are most suitable for a rapidly changing climate. And the clearest path to resilience is diversity. The notion of planning and planting diverse tree communities for resilience to change has precedent. In reaction to the devastating effects of diseases and pests that destroyed virtually every American elm and American chestnut (and other species), a rule of thumb called the 10/20/30 rule has long been suggested: no more than 10% of plantings should be within a single species, no more than 20% within a single genus; and no more than 30% within a single family (original article). The risk of a disease or pest wiping out every tree is greatly reduced.

To hedge against the new “pest” of climate change perhaps we can sprinkle a mindfulness of provenance into our genetically diverse plantings. That is, (in the Northern Hemisphere) let’s deliberately move some plants from South to North. So, my oak tree may indeed have more to give. And maybe my bald cypress, which seems to thrive as well on a well-drained Iowa hillside as in a Louisiana swamp. Or my Osage Orange ‘White Shield,’ plucked from arid western Oklahoma, but from a species which achieves its highest densities in repeatedly flooded “bodark swamps.”

Recent Literature on Assisted Migration

2016

“As hardiness zones continue to migrate northward with climate change, heat island mitigation and other environmental management strategies employing green infrastructure must identify tree species that are likely to remain well adapted to urban climates many years into the future.”

“. . . uncertainty exists around future water availability than future temperatures. Planting forests with high genetic diversity, including seedlings from warmer, drier populations, should provide some buffering against this uncertainty as long as sites remain within a species’ bioclimatic niche.”

“Ultimately, implementation of assisted migration, or other large scale conservation mechanisms, will require reconciliation between the hubris of being able to control nature, with the hubris that humans are somehow not a part of nature.”